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A Review of Glass Ionomer Restorations in the Primary Dentition

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C L I N I C A L P R A C T I C E A Review of Glass Ionomer Restorations in the Primary Dentition ã Shiu-yin Cho, BDS, MDS ã ã Ansgar C. Cheng, BDS, MS ã A b s t r a c t Glass ionomer cements are tooth-coloured materials that bond chemically to dental hard tissues and release fluoride for a relatively long period. They have therefore been suggested as the materials of choice for the restoration of carious primary teeth. However, the clinical performance of conventional and metal-reinforced gl
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  October 1999, Vol. 65, No. 9  491 Journal of the Canadian Dental Association  R estoring carious teeth is one of the major treatmentneeds of young children. A restoration in the primarydentition is different from a restoration in the perma-nent dentition due to the limited lifespan of the teeth and thelower biting forces of children. 1,2 As early as 1977, it was sug-gested that glass ionomer cements could offer particular advan-tages as restorative materials in the primary dentition becauseof their ability to release fluoride and to adhere to dental hardtissues. 3 And because they require a short time to fill the cavity,glass ionomer cements present an additional advantage whentreating young children. 2 Basic Chemistry In general, glass ionomer cements are classified into threemain categories: conventional, metal-reinforced and resin-modified. 4-7 Conventional glass ionomer cements were firstintroduced in 1972 by Wilson and Kent. 8 They are derivedfrom aqueous polyalkenoic acid such as polyacrylic acid and aglass component that is usually a fluoroaluminosilicate. Whenthe powder and liquid are mixed together, an acid-base reactionoccurs. As the metallic polyalkenoate salt begins to precipitate,gelation begins and proceeds until the cement sets hard. 4,5 Recently, several faster setting, high-viscosity conventionalglass ionomer cements have become available. Called viscousor condensable glass ionomer cements by some authors, 9 theserestorative materials were srcinally developed in the early1990s for use with the atraumatic restorative treatment insome developing countries. 10 These materials set faster and areof higher viscosity because of finer glass particles, anhydrouspolyacrylic acids of high molecular weight and a high powder-to-liquid mixing ratio. 9,10 The setting reaction is the same asthe acid-base reaction typical of conventional glass ionomercements.Metal-reinforced glass ionomer cements were first intro-duced in 1977. The addition of silver-amalgam alloy powderto conventional materials increased the physical strength of thecement and provided radiopacity. 11 Subsequently, silver parti-cles were sintered onto the glass, and a number of productsthen appeared where the amalgam alloy content had been fixedat a level claimed to produce optimum mechanical propertiesfor a glass cermet cement. 11,12 In 1992, resin-modified glass ionomer cements were devel-oped that could be light cured. In these materials, the funda-mental acid-base reaction is supplemented by a second resinpolymerization usually initiated by a light-curing process. 6,7 Intheir simplest form, they are glass ionomer cements that con-tain a small quantity of a water-soluble, polymerizable resincomponent. More complex materials have been developed bymodifying the polyalkenoic acid with side chains that couldpolymerize by light-curing mechanisms in the presence of photo initiators, but they remain glass ionomer cements bytheir ability to set by means of the acid-base reaction. 6 A Review of Glass Ionomer Restorationsin the Primary Dentition ã Shiu-yin Cho, BDS, MDS ãã Ansgar C. Cheng, BDS, MS ã A b s t r a c t Glass ionomer cements are tooth-coloured materials that bond chemically to dental hard tissues and release fluoride for a relatively long period. They have therefore been suggested as the materials of choice for the restora- tion of carious primary teeth. However, the clinical performance of conventional and metal-reinforced glass ionomer restorations in primary molars is disappointing. And although the handling and physical properties of the resin-modified materials are better than their predecessors, more clinical studies are required to confirm their efficacy in the restoration of primary molars. MeSH Key Words:  dental restoration, permanent/methods; dentition, primary; glass ionomer cements.  ©  JCan Dent Assoc 1999; 65:491-5 This article has been peer reviewed. C L I N I C A L P R A C T I C E  Advantages Glass ionomer cements exhibit a number of advantagesover other restorative materials.  Adhesion By bonding a restorative material to tooth structure, thecavity is theoretically sealed, protecting the pulp, eliminatingsecondary caries and preventing leakage at the margins. Thisalso allows cavity forms to be more conservative and, to someextent, reinforces the remaining tooth by integrating restora-tive material with the tooth structures. 13 Bonding between thecement and dental hard tissues is achieved through an ionicexchange at the interface. 4,14 Polyalkenoate chains enter themolecular surface of dental apatite, replacing phosphate ions.Calcium ions are displaced equally with the phosphate ions soas to maintain electrical equilibrium. 5 This leads to the devel-opment of an ion-enriched layer of cement that is firmlyattached to the tooth. 14 The shear bond strength of conventional glass ionomercements to conditioned enamel and dentin is relatively low,varying from 3 to 7 MPa. 7,13 However, this bond strength ismore a measure of the tensile strength of the cement itself,since fractures are usually cohesive within the cement, leavingthe enriched residue attached to the tooth. 5 Comparisonsbetween resin-modified glass ionomer cements and conven-tional materials reveal that the shear bond strength of the for-mer is generally greater, 15 but that they show very low bondstrength to unconditioned dentin compared to conventionalmaterials. 13 Conditioning therefore plays a greater role inachieving effective bonding with the resin-modified glassionomer cements. In addition, when the enamel surface isetched with phosphoric acid, the bond strength of the resin-modified materials is close to that of composite-resin bondedto etched enamel. 16 This suggests, along with the effectsoflight-curing, that the bonding mechanism of resin-modifiedglass ionomer cements may be different from that of conventional materials.  Margin Adaptation and Leakage The coefficient of thermal expansion of conventional glassionomer cements is close to that of dental hard tissues and hasbeen cited as a significant reason for the good margin adapta-tion of glass ionomer restorations. 4,7 Even though the shearbond strength of glass ionomer cements does not approachthat of the latest dentin bonding agent, glass ionomer restora-tions placed in cervical cavities are very durable. 7 Nevertheless,microleakage still occurs at margins. An in vitro study hasshown that conventional glass ionomer cements were less reli-able in sealing enamel margins than composite-resin. 17 Theyalso failed to eliminate dye penetration at the gingival mar-gins. 17-19 Although resin-modified glass ionomer cementsshow higher bond strength to dental hard tissues than conven-tional materials, they exhibit variable results in microleakagetests. 20-22 Not all of them display significantly less leakageagainst enamel and dentin than their conventional counter-parts. 20,22 This may be partly because their coefficient of thermal expansion is higher than conventional materials,though still much less than composite-resins. 6,7 Controversyalso exists as to whether the slight polymerization shrinkage issignificant enough to disrupt the margin seal. 6  Fluoride Release Fluoride is released from the glass powder at the time of mixing and lies free within the matrix. It can therefore bereleased without affecting the physical properties of thecement. 23 Since it can also be taken up into the cement duringtopical fluoride treatment and released again, the cement mayact as a fluoride reservoir over a relatively long period. 24 As aresult, it has been suggested that glass ionomer cements will beclinically cariostatic. 25 This assumption is supported by somein vitro studies using an artificial caries model in which lessdecalcification has been found in cavities restored with glassionomer cements. 26,27 The amount of constant fluoride releasedid not differ much between brands of conventional glassionomer cements. 28 The fluoride release of some resin-modi-fied materials is at least the same as conventional materials butvaries amongst different commercial products. 28,29 Nevertheless, the critical amount of fluoride released from arestoration that is required to be effective in inhibiting carieshas not yet been established.Despite the constant fluoride release of glass ionomerrestorations, results from clinical studies are not so promising.Kaurich and others 30 compared glass ionomer and composite-resin restorations over one year and concluded that there waslittle clinical advantage in using glass ionomer cement. Tyas 31 examined cervical composite-resin and glass ionomer restora-tions five years after placement and found no significant dif-ference in recurrent caries rates. More clinical studies wouldtherefore be needed to confirm the cariostatic effect of glassionomer cements.  Esthetics Conventional glass ionomer cements are tooth-colouredand available in different shades. Although the addition of resin in the modified materials has further improved theirtranslucency, they are still rather opaque and not as esthetic ascomposite-resins. In addition, surface finish is usually not asgood. The colour of resin-modified materials has been report-ed to vary with the finishing and polishing techniques used. 32 Potential also exists for increased body discolouration and sur-face staining because of their hydrophilic monomers andincomplete polymerization. 33 Nevertheless, the demand foresthetics in the primary dentition is usually lower than in thepermanent dentition.  Biocompatibility The biocompatibility of glass ionomer cements is veryimportant because they need to be in direct contact withenamel and dentin if any chemical adhesion is to occur. In anin vitro study, freshly mixed conventional glass ionomercement was found to be cytotoxic, but the set cement had noeffect on cell cultures. 34 In another study, the pulpal response Journal of the Canadian Dental Association  492  October 1999, Vol. 65, No. 9 Cho, Cheng   to glass ionomer cements in caries-free human premolarsplanned for extraction was examined. 35 The result showed thatalthough glass ionomer cement caused a greater inflammatoryresponse than zinc-oxide eugenol cement, the inflammationresolved spontaneously with no increase in reparative dentinformation. More recently, Snugs and others 36 have evendemonstrated dentin bridging in monkey teeth wheremechanical exposures in otherwise healthy pulps were cappedwith a glass ionomer liner. Therefore, lining is normally notnecessary under conventional glass ionomer restorations whenthere is no pulpal exposure. 5 Concern has been raised regarding the biocompatibility of resin-modified materials since they contain unsaturatedgroups. A cell culture study revealed poor biocompatibility of a resin-modified liner. 37 In contrast, Cox and others 38 showedthat a resin-modified glass ionomer cement did not impairpulp healing when placed on exposed pulps. As a result of thisuncertainty, use of resin-modified materials in deep unlinedcavities is probably not advisable. 6 Disadvantages The use of glass ionomer cements canhave limitations in very specific circum-stances.  Physical Strengths The main limitation of the glass ionomercements is their relative lack of strength andlow resistance to abrasion and wear.Conventional glass ionomer cements havelow flexural strength but high modulus of elasticity, and are therefore very brittle and prone to bulk frac-ture. 39 Some glass cermet cements are arguably stronger thanconventional materials but their fracture resistance remainslow. 9,11 The resin-modified materials have been shown to havesignificantly higher flexural and tensile strengths and lowermodulus of elasticity than the conventional materials. 39,40 They are therefore more fracture-resistant but their wear resis-tance has not been much improved. 33,39 In addition, theirstrength properties are still much inferior to those of compos-ite-resins, and so should notbe subject to undue occlusal loadunless they are well supported by surrounding toothstructure. 6,33,39 Water Sensitivity Conventional glass ionomer restorations are difficult tomanipulate as they are sensitive to moisture imbibition duringthe early setting reaction and to desiccation as the materialsbegin to harden. Although it was believed that the occurrenceof the resin polymerization in the modified materials reducesthe early sensitivity to moisture, 23 studies have shown that theproperties of the materials changed markedly with exposure tomoisture. 41 Whether it is necessary to place protective cover-ing on resin-modified glass ionomer restorations remainscontroversial. 6,21,41 Clinical Success in Primary Molars Clinical trials investigating the longevity of glass ionomerrestorations in primary molars are mostly short-term studies of less than three years. The longest survival rates for glassionomer restorations are in low stress areas such as Class IIIand Class V restorations. 23 In an early study, Vlietstra andothers 42 reported that 75% of conventional glass ionomerrestorations in primary molars were intact after one year, andthat margin adaptation, contour and surface finish were allsatisfactory. The longest clinical study has been conducted byWalls and others 43 who compared conventional glass ionomerrestorations with amalgam restorations in primary molars.Although they reported no significant difference in overall fail-ure rates after two years, follow-up of the restorations up tofive years showed that glass ionomer restorations had signifi-cantly inferior survival time to amalgam. 44 The importance of long-term clinical studies should therefore not be overlooked.Other short-term trials also show poor success rates of con-ventional glass ionomer restorations inprimary molars. Ostlund and others 45 compared Class II restorations of amal-gam, composite-resin and glass ionomercement in primary molars and reported ahigh failure rate for glass ionomer cementof 60% after one year. In contrast, thefailure rates for amalgam and composite-resin restorations were eight and 16%respectively. Fuks and others 46 comparedthe clinical performance of a glassionomer cement with amalgam inClassII restorations in primary molars.Only nine of 101 glass ionomer restorations met all qualitycriteria after one year, whereas 90% of the amalgam restora-tions met all the evaluation criteria after three years.Papathanasiou and others 47 investigated the mean survivaltime of different types of restorations in primary molars andfound that the mean survival time for glass ionomer restora-tions was only 12 months compared to more than five yearsfor stainless steel crowns and amalgam restorations. In a recentstudy, the median survival time for Class II glass ionomerrestorations in primary molars was also reported to be signifi-cantly shorter than for amalgam restorations. 48 The results of these studies indicate that conventional glass ionomer cementis not an appropriate alternative to amalgam in the restorationof primary molars unless the teeth are expected to exfoliate inone or twoyears.Short-term clinical studies have shown that the perfor-mance of Class II glass cermet restorations in primary molarsis significantly worse than conventional materials. 1,49 Although Hickel and Voss 2 found no significant difference inthe cumulative failure rates between glass cermet and amalgamrestorations in primary molars, they did find that the loss of anatomical form was more severe with glass cermet cement,concluding that amalgam should be preferred in restorationswith occlusal stress.Only limited data are available for resin-modified glass October 1999, Vol. 65, No. 9  493  Journal of the Canadian Dental Association AReview of Glass Ionomer Restorations in the Primary Dentition  The main limitation of  the glass ionomer cements is their relative lack of strength and low resistance to abrasion and wear.  ionomer restorations in primary molars and they are mostly inthe form of clinical experience 50 or abstracts. 51,52 The initialresults show that these restorations perform better than con-ventional materials in short-term comparisons. 51,52 Long-termtrials would be required to confirm their efficacy. Until then,the choice of resin-modified glass ionomer restorations in pri-mary molars remains a relatively empirical one and shouldtherefore be restricted to cavities well supported bysurrounding tooth structures, such as small Class I and ClassIIrestorations. In cases where high occlusal load is expected,other alternatives such as amalgam or stainless steel crownsshould be considered. Conclusion The desirable properties of glass ionomer cements makethem useful materials in the restoration of carious lesions inlow stress areas such as smooth surface and small anterior prox-imal cavities in primary teeth. Results from clinical studies,however, do not support the use of conventional or metal-reinforced glass ionomer restorations in primary molars. Moreclinical studies are required to confirm the efficacy of resin-modified glass ionomer restorations in primary molars.  Dr. Cho is a dental officer in the Department of Health, HongKong.  Dr. Cheng is head of maxillofacial prosthetics at the OntarioCancer Institute - Princess Margaret Hospital, Toronto, Ont.  Reprint requests to:  Dr. Shiu-yin Cho, Tuen Mun School Dental Clinic, 16 Tsun Wen Road, Tuen Mun, Hong Kong.The authors have no declared financial interest in any companymanufacturing the types of products mentioned in this article. References 1. Kilpatrick NM, Murray JJ, McCabe JF. The use of a reinforced glass-ionomer cermet for the restoration of primary molars: a clinical trial.  BrDentJ  1995; 179:175-9.2. Hickel R, Voss A. A comparison of glass cermet cement and amalgamrestorations in primary molars.  ASDC J Dent Child  1990; 57:184-8.3. McLean JW, Wilson AD. The clinical development of glass ionomercement. II. Some clinical applications.  Aust Dent J  1977; 22:120-7.4. 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